1. Field of the Invention
The present invention relates to a red (oscillation wavelength: 650-nm band) and infrared (oscillation wavelength: 780-nm band) integrated high-power two-wavelength semiconductor laser device that is used as, for example, a light source for a pickup of an optical disc device, a light source for other electronic devices and information processing devices, or the like, and a method for fabricating the two-wavelength semiconductor laser device.
2. Description of the Related Art
On the current market, there are a large-capacity Digital Video Disc (DVD) capable of high-density recording and various DVD devices that record and reproduce the disc. Taking it into account how popular DVD devices have become (e.g., DVD devices are becoming widespread as DVD recorders in homes and are often supplied as standard equipment to personal computers), the demand is expected to increase more and more in the future. On the other hand, a function of recording and reproducing a Compact Disc (CD), which has already become widespread, is simultaneously required. Therefore, it is essential to provide a capability to record and reproduce both DVDs and CDs. At present, a semiconductor laser with 650-nm band oscillation wavelengths having an active layer made of (AlxGa1−x)yIn1−yP mixed crystal (0≦x≦1, 0≦y≦1) is employed as a laser light source for recording and reproduction of DVDs, while a semiconductor laser with 780-nm band oscillation wavelengths having an active layer made of AlxGa1−xAs mixed crystal (0≦x≦1) is employed as a laser light source for recording and reproduction of CDs.
In recent years, there is also an increasing demand for LightScribe (a technology that utilizes laser light energy to create text or graphics on a disc surface), so that higher power is required for a semiconductor laser for recording and reproducing CDs.
For a Blu-ray Disc (BD), which is expected as a high-density and large-capacity disc, a semiconductor laser with 400-nm band oscillation wavelengths is employed. It is also contemplated that a single optical system is used for both DVDs and CDs. When a semiconductor laser for BDs is mainly used, a semiconductor laser for DVDs and CDs needs to have higher power.
Incidentally, the market demands a rapid reduction in cost. To meet the demand, there is a desire for a lower-cost optical pickup included in a semiconductor laser device. There are generally two methods for reducing the cost of an optical pickup. Firstly, parts constituting an optical pickup are simplified (reduced) and their cost is reduced. A semiconductor laser is a key component and cannot be removed, but its cost can be reduced by simplifying its structure and fabrication process. Secondly, the yield of an optical pickup is improved. A cause for a decrease in yield is that semiconductor lasers for DVDs and CDs are conventionally separately arranged, so that a complicated step of optically adjusting an optical pickup is required. To avoid this, a two-wavelength laser is recently employed in which semiconductor lasers for DVDs and CDs are monolithically integrated on a substrate. Also, when a material for an optical pickup may be changed from a metal to a less expensive resin material by simplifying the optical pickup, the resultant structure may be often disadvantageous to heat radiation. It is also becoming important to improve temperature characteristics of a semiconductor laser.
Under these circumstances, there is a demand for an integrated two-wavelength semiconductor laser that has a CW light output exceeding 200 mW for each of a DVD and a CD. Meanwhile, there is also an increasing demand for a reduction in cost.
FIGS. 10A and 10B show structures of conventional integrated two-wavelength semiconductor laser devices.
In the structure of FIG. 10A, a semiconductor laser for CDs (semiconductor laser device 200A) and a semiconductor laser for DVDs (semiconductor laser device 200B) are monolithically fabricated on the same n-type GaAs substrate 100. The CD semiconductor laser device 200A is made of AlGaAs mixed crystal, while the DVD semiconductor laser device 200B is made of AlGaInP mixed crystal. Specifically, the CD semiconductor laser device 200A comprises a cladding layer 101 made of n-type AlGaAs, a light guide layer 102 made of AlGaAs, a quantum well active layer 103 made of AlGaAs, a light guide layer 104 made of AlGaAs, a cladding layer 105 made of p-type AlGaAs, an etch stop layer 106, a cladding layer 107 made of p-type AlGaAs (ridge portion), a current confining layer 108, and ohmic electrodes 109 and 119. On the other hand, the DVD semiconductor laser device 200B comprises a cladding layer 110 made of n-type AlGaInP, a light guide layer 111 made of AlGaInP, a quantum well active layer 112 made of AlGaInP, a light guide layer 113 made of AlGaInP, a cladding layer 114 made of p-type AlGaInP, an etch stop layer 115, a cladding layer 116 made of p-type AlGaInP (ridge portion), a current confining layer 117, and ohmic electrodes 118 and 119.
In a conventional two-wavelength semiconductor laser device having such a structure, the active layer structure of the high-power laser generally often has a multi-quantum well structure, and the Al molar ratios of the guide layer and the barrier layer are each generally set to have a value intermediate between those of the cladding layer and the well layer and as low as possible. This is because a layer containing a large amount of Al, which is highly reactive, tends to have a decreased level of crystallinity, so that the light emission efficiency of the active layer is highly likely to decrease. Therefore, a general guide layer and barrier layer have an Al molar ratio of about 0.3 in the CD semiconductor laser device and about 0.5 in the DVD semiconductor laser device. The guide layer and the barrier layer are often set to have the same Al molar ratio, taking into account the stabilization and simplification of a crystal growth process.
The structure of FIG. 10B is the same as that of FIG. 10A, except that the cladding layer included in the CD semiconductor laser device is made of AlGaInP mixed crystal (see, for example, Japanese Unexamined Patent Application Publication Nos. 2002-111136 and 2005-109102 (hereinafter referred to as Patent Documents 1 and 2)). Specifically, cladding layers 101a and 107a of the CD semiconductor laser device 200A are made of n-type AlGaInP and p-type AlGaInP, respectively. By employing AlGaInP mixed crystal having a large band gap as the cladding layers as in this structure, it is possible to suppress overflow of carriers from the active layer, resulting in higher power of the device.
On the other hand, in a semiconductor laser device having a Fabry-Perot type optical resonator structure, light output is often limited by Catastrophic Optical Damage (COD) at a light emitting facet. To prevent this, it is essential to provide a facet window structure (e.g., reference numeral 120 indicates a facet window structure in each of the structures of FIGS. 10A and 10B). The facet window structure selectively diffuses an impurity in the quantum well active layer in the vicinity of the light emitting facet so that the quantum well active layer has average composition, thereby expanding a band gap in the vicinity of the light emitting facet. As a result, light absorption in the vicinity of the light emitting facet can be reduced, so that COD can be suppressed.
As an impurity used for formation of such a facet window structure, Zn is generally used. It has been reported that a quantum well active layer having average composition can be generally relatively easily achieved in a semiconductor laser device made of AlGaInP mixed crystal. It has also been reported that, when Zn is used, the quantum well active layer can be caused to have average composition by thermal diffusion at about 600° C., and the COD level can be improved (see, for example, IEEE J. of Quantum Electronics, Vol. 29, No. 6, June 1993 pp. 1874-1879 and Jpn. J. Appl. Phys. Vol. 36 (1997) pp. 2666-2670 (hereinafter referred to as Non-Patent Document 1)).
On the other hand, since the diffusion rate of Zn is lower in AlGaAs mixed crystal than in AlGaInP mixed crystal, it is not easy to obtain average composition of the quantum well active layer by Zn diffusion in the semiconductor laser device 200A made of AlGaAs mixed crystal of FIG. 10A. In this regard, for example, IEEE J. of Quantum Electronics, Vol. 26, No. 5, May 1990 pp. 837-842 (hereinafter referred to as Non-Patent Document 2) introduces an example in which Zn diffusion is performed twice (double diffusion method) in the CD semiconductor laser device made of AlGaAs mixed crystal so that Zn can be diffused to the active layer. In this case, however, the diffusion temperature reaches as high as 900° C., indicating that it is difficult to diffuse Zn to AlGaAs mixed crystal.
A structure has also been proposed in which Si is used as an impurity when the facet window structure is formed. In this case, Si is introduced as an impurity by an ion implantation technique after crystal growth of a portion of the cladding layer on the quantum well active layer (see, for example, Japanese Unexamined Patent Application Publication No. 2002-185077). In this case, the remaining semiconductor layers, such as a cladding layer and the like, need to be grown again after formation of a window structure, resulting in a complicated fabrication process.